1. Field of the Invention
The present invention relates to a lead-free solder alloy, which is used in, for example, printed circuit boards treated with nickel (Ni)/gold (Au), and a manufacturing method thereof. More specifically, the present invention relates to: a lead-free solder alloy, which has a melting point similar to those of prior lead-free solder alloys, excellent wettability, very low segregation ratio, and excellent weldability with a welding base metal, such that it improves temperature cycle performance and drop impact resistance simultaneously, when it is applied to electronic devices and printed circuit boards; a method for manufacturing said lead-free solder alloy; and electronic devices and printed circuit boards which include said lead-free solder alloy.
2. Description of the Prior Art
Recently, as portable digital devices having a small and thin design have gained in popularity, semiconductor packages mounted therein also have become thinner. Semiconductor packages serve to electrically connect semiconductor chips formed on wafers and seal and package the semiconductor chips, such that the semiconductor chips can be used by users in daily life. As portable digital devices have become high-performance and multifunctional, the number of semiconductor chips mounted therein have increased, whereas the entire size of the devices has become smaller.
Accordingly, reliability standards for solder joints applied in such electronic parts and portable digital devices have become significantly higher and more diverse. Particularly, as the use of portable digital devices has rapidly increased, the demand for products which simultaneously satisfy the temperature cycle performance and drop impact performance of solder joints has rapidly increased.
Lead-free solder alloys, which have been used in the prior art, are mostly ternary alloys based on tin (Sn), silver (Ag) and copper (Cu). As the content of silver (Ag) in such ternary alloys increases, the drop impact performance of the alloys shows a tendency to deteriorate, whereas the temperature cycle performance shows a tendency to improve. On the contrary, as the content of silver (Ag) decreases, the drop impact performance shows a tendency to improve whereas the temperature cycle performance shows a tendency to deteriorate.
Because solder joints provided with lead-free solder alloys serve to maintain mechanical strength and also function as heat diffusion channels, buffers against heat shocks, and channels providing electric flow, the solder joints require high reliability. Accordingly, studies on reactions between solders and substrates or between solders and printed circuit boards (PCBs) have been conducted for a long time.
In such studies, the most important issue occurring in joints between solders in lead-free solder alloys and substrates or between solders and printed circuit boards is how the reliabilities of solder joints, that is, thermal fatigue life, tensile strength and fracture toughness, are increased through the production and control of intermetallic compounds. However, the reliability of solder joints also definitely differs depending on the surface treatment of substrates and printed circuit boards.
In the prior art, lead-free solder alloys were generally used regardless of the surface treatment of substrates and printed circuit boards. During a reflow soldering process, intermetallic compound layers form intermetallic compounds which differ depending on the surface of the solders and substrates or the surface of solders and printed circuit boards, and the solder joint reliability varies depending on the intermetallic compound layers. The intermetallic compound layers produced by the wetting phenomena between solders and substrates and between solders and packages, grow at temperatures lower than the melting points thereof through solid-phase reactions. Herein, the growth of the intermetallic compound layers is a function of temperature and time, and the morphology of solder joints is made so as to minimize interfacial and grain boundary energy.
Accordingly, because of the above-described differing properties of solder joints, alloys which have good drop impact performance and a relatively low silver (Ag) content have been developed. However, there are problems in that, because the tin (Sn) content relatively increases with decreases in the silver (Ag) content, the probability to form intermetallic compound layers increases, and the growth of the intermetallic compound layers deteriorate thermal fatigue life, that is, temperature cycle performance, among solder joint reliabilities. In consideration of a current trend in which the use of portable electronic devices is rapidly increasing, the above alloys do not satisfy the demand for products which must satisfy the temperature cycle performance and drop impact performance of solder joints.